FIG. 6 shows a prior organic electro-luminescence (EL) system which is activated by using a TFT active matrix drive circuit, in which the numeral 10 is a display screen having image cells 10-1, 10-2, 10-3, 10-4, et al, the numeral 15 is a shift register which receives an X-axis signal, and the numeral 16 is a shift register which receives a Y-axis signal. The power supply voltage E.sub.0 for operating organic electro-luminescence (EL) is applied to the screen display 10. A picture cell 10-1 has an organic electro luminescence element EL.sub.1 which is produced through thin film technique for lighting, a bias TFT (thin film transistor) 11-1 for controlling lighting of said electro-luminescence element EL.sub.1, a capacitor C.sub.1 coupled with the gate electrode of said bias TFT 11-1, and a Y-axis select switch 12-1 which writes a signal into said capacitor C.sub.1. Other picture cells 10-2, 10-3, 10-4 et al are similar to the picture cell 10-1.
The Y-axis select switch 12-1 is made of a TFT (thin film transistor), with a gate electrode coupled with the terminal Y.sub.1 of the shift register 16. The Y-axis select switch 12-1 is further coupled with the X-axis select switch 13, which is made of a TFT with a gate electrode coupled with the terminal X.sub.1 of the shift register 15. The X-axis select switch 13 receives an image data signal D.sub.i.
When a synchronization signal is provided to the terminal Y.sub.1 in the Y-axis shift register 16, the Y-axis select switches 12-1, 12-2, et al are turned ON. Simultaneously, when a synchronization signal is provided to the terminal X.sub.1 in the X-axis shift register 15, the X-axis select switch 13 is turned ON, and then, an image data signal D.sub.1 applied to the X-axis select switch 13 is kept in the capacitor C.sub.1 through the Y-axis select switch 12-1.
Next, when a synchronization signal on the terminal X.sub.1 is OFF, and it is ON on the terminal X.sub.2, the X-axis select switch 13 turned OFF, and the X-axis select switch 14 is turned ON, and an image data signal D.sub.2 applied to the X-axis select switch 14 is kept in the capacitor C.sub.2 through the Y-axis select switch 12-2. Thus, the Y-axis select switches 12-1, 12-2 et al function as a select switch for storing charge in a capacitor C.sub.1, C.sub.2, et al depending upon image data signal.
Thus, an image data signal D.sub.1, D.sub.2, et al is stored in a capacitor C.sub.1, C.sub.2, et al, respectively, and, a bias TFT (thin film transistor) 11-1, 11-2, et al is turned ON so that an organic EL element EL.sub.1, EL.sub.2, et al emits light according to an image data signal D.sub.1, D.sub.2, et al. After the picture cells 10-1, 10-2 et al which relates to the terminal Y.sub.1 operate for light emission, the synchronization signal at the terminal Y.sub.1 is turned OFF, and the synchronization signal at the terminal Y.sub.2 is turned ON so that the picture cells 10-3, 10-4 et al operates for light emission.
Above operation of an EL image display system is shown for instance in I.E.E.E. Trans. Electron Devices, Vol. ED-22, No.9, September, 1975 (pages 739-748).
FIG. 7 shows another prior image display system, which is called Alt-Plesko technique for passive matrix display in which each line or row is activated sequentially one by one.
In FIG. 7, organic EL elements E.sub.11, E.sub.12, E.sub.21, and E.sub.22 are connected as shown in the figure, the rows R.sub.1 (ROW-1) and R.sub.2 (ROW-2) are supplied with a pulse voltage V volt or 0 volt through a current restriction resistor R, and columns C.sub.1 (Column 1) and C.sub.2 (Column 2) are supplied with voltage 0 volt or V volt.
It is assumed that the elements E.sub.11,E.sub.12, and E.sub.21 are bright, and the element E.sub.22 is dark.
It is supposed at time t.sub.1 that the row-1 is supplied with the voltage V, the row R.sub.2 is grounded, and the columns C.sub.1 and C.sub.2 are supplied with the voltage 0 volt, then, the elements E.sub.11 and E.sub.12 which are coupled with the row 1 are bright, and the elements E.sub.21 and E.sub.22 are dark.
Then, it is supposed at time t.sub.2 that the row 1 is grounded, the row 2 is supplied with the voltage V, the column C.sub.1 is supplied with the voltage 0, and the column C.sub.2 is supplied with the voltage V. Then, the element E.sub.21 is bright, and the elements E.sub.11, E.sub.12 and E.sub.22 are dark.
If the above operation is repeated quickly with the repetition frequency 60 Hz which a human vision can not follow, it appears that the elements E.sub.11, E.sub.12 and E.sub.21 are bright, and the element E.sub.22 is dark.
As mentioned above, an organic EL element is selectively activated by supplying the voltage V to each rows sequentially, and supplying the voltage 0 or V to each columns.
The luminance or light intensity of a bright element in that case is the time average of the luminance when it is bright, and the average luminance in FIG. 7 which has two rows is half of the luminance of a cell when it is bright.
However, prior arts in FIGS. 6 and 7 have the disadvantages as follows.
A prior art of FIG. 6 has the disadvantage that each picture cell must be provided with a plurality of TFTs (thin film transistor), and therefore, the total number of TFTs is extravagant. Therefore, when a display screen is large, it is not practical to use a TFT in view of investment and yield rate of production.
A prior art of FIG. 7 has the disadvantage that the average luminance of a bright cell is decreased when a number of rows is large. Therefore, when some average luminance is designed, each cell must emit strong light, which needs high voltage to be applied. Therefore, the maximum number of rows and/or luminance of each cell is restricted by a withstand voltage and/or life time of an image cell.